AbstractThe structure–property relationship of polyester polyols in cationically polymerized, amorphous epoxy-based copolymers is investigated. An epoxy resin is polymerized in the presence of structurally different polyesters. These resulting copolymers show improved tensile strength and toughness. The optimal epoxide/polyester ratio depends on the structure of the polyesters. Poly(d-valerolactone)(PVL) reveals the highest ester group density of the investigated polyesters, which enhances physical interactions with the epoxide during polymerization as well as in the network. Furthermore, PVL leads to outstanding tensile strength, strain at break, and toughness. Among all polyester polyols examined, PVL leads to the highest gel fraction or, in other words, the most complete integration into the epoxy network. This work shows that polyesters that are present in the reactive system should be covalently integrated into the polymer network as completely as possible to obtain good mechanical properties of the amorphous copolymer.